1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a micromirror actuator and a method of manufacturing the same and, more particularly, to a micromirror actuator having a micromirror, which is made to rise to a precisely vertical state or is maintained in a horizontal state using an electrostatic force with a low voltage and wherein an electrostatic force opposite to the driving force of the micromirror is blocked, and a method of manufacturing the same.
2. Description of the Related Art
Generally, optical switches are capable of selecting an optical route and thus allowing an optical signal to be transmitted from an input terminal to a predetermined output terminal. Referring to FIG. 1, a conventional optical switch comprises a plurality of micromirror actuators 10 arranged in a two-dimensional matrix. Light emitted from an optical fiber 43 of an input unit is collimated into a parallel beam through a micro lens 45 which is a focal distance apart from the optical fiber 43. The parallel beam is incident upon a corresponding one of vertically oriented micro mirrors 31a, 31b, 31c and 31d and then is reflected. The reflected parallel beam enters an output unit, passes through a micro lens 46, and is transmitted to an optical fiber 48 of an output unit. Such an optical switch is capable of selecting an optical route by reflecting an incident optical signal using vertically oriented micromirrors 31a through 31d and letting an incident optical signal pass over horizontally oriented micromirrors 32. For example, as shown in FIG. 1, the micromirror 31a placed at the first row from the top and the fourth column from the left, the micromirror 31b placed at the second row from the top and the third column from the left, the micromirror 31c placed at the third row from the top and the first column from the left, and the micromirror 31d placed at the fourth row from the top and the second column from the left are made to stand vertically and the other micromirrors 32 are maintained to be horizontal, thereby transmitting an optical signal via a desired optical route.
FIG. 2 illustrates a conventional micromirror actuator 10 taking advantage of electrostatic force. Referring to FIG. 2, a trench 5 is formed in a substrate 15, and supporting posts 20 stand straight at opposite ends of the trench 5. A torsion bar 25 is supported by the supporting posts 20, and a micromirror 30 is coupled to the torsion bar 25 so as to be capable of rotating but elastically biased to a horizontal state. The micromirror 30 is comprised of a driving unit 30a, which faces the trench 5 when the micromirror 30 is in a horizontal state, and a reflecting unit 30b opposite to the driving unit 30a with the torsion bar 25 formed therebetween.
FIG. 3 is a cross-sectional view of the conventional micromirror actuator 10 taken along the line 3—3 in FIG. 2. Referring to FIG. 3, a lower electrode 37 is installed at the bottom of the trench 5 and a side electrode 40 is installed at one sidewall of the trench 5. The micromirror 30 is driven by an electrostatic force induced by interaction between the lower and side electrodes 37 and 40 and the driving unit 30a. In other words, if an attractive electrostatic force acts between the lower electrode 37 and the driving unit 30a, the micromirror 30 rotates clockwise about the torsion bar 25. As the micromirror 30 rotates, an attractive electrostatic force between the driving unit 30a and the side electrode 40 increases in strength causing the micromirror 30 to continue rotating until it stands vertically. After the electrostatic driving force is removed, the micromirror 30 is restored to a horizontal state due to the elastic restoring force of the torsion bar 25.
In this case, an electrode surface, upon which an electrostatic force acts, is formed on the whole micromirror 30. Thus, if voltage is applied to the micromirror 30, an electrostatic force acts between the driving unit 30a and the lower and side electrodes 37 and 40 and between the reflecting unit 30b and the lower and side electrodes 37 and 40. The distance from the reflecting unit 30b to the lower and side electrodes 37 and 40 is greater than the distance from the driving unit 30a to the lower and side electrodes 37 and 40; however, the substrate 15 is formed of silicon and the dielectric constant of silicon is at least 10 times greater than the dielectric constant of air. Thus, an electrostatic force can be strongly exerted between the reflecting unit 30b and the side and lower electrodes 40 and 37 through the silicon substrate 15.
Accordingly, if the electrostatic force between the driving unit 30a and the side and lower electrodes 40 and 37 is referred to as f1 and the electrostatic force between the reflecting unit 30b and the side and lower electrodes 40 and 37 is referred to as f2, an electrostatic force f3 actually contributing to driving the micromirror 30 is equal to f1-f2. In other words, when driving the micromirror 30 through the driving unit 30a, the electrostatic force introduced by interaction between the reflecting unit 30b and the side and lower electrodes 40 and 37 acts upon the micromirror 30 in a direction opposite to the direction of the driving force of the micromirror 30, and thus the driving force introduced by the driving unit 30a is inhibited. As a result, a driving voltage required to drive the micromirror 30 increases, and it is difficult to control the micromirror 30 to stand precisely vertically because of the opposite electrostatic force introduced by the reflecting unit 30b. 